Moonrise and Moonset

The moon orbits the earth completing one orbit about every 28 days (Sources vary between 27.32 and 29.53 days for one orbit depending on how the beginning and ending of the orbit are measured.). Like the earth’s orbit of the sun, the moon’s orbit of the earth is elliptical (oval) rather than circular. When viewed in three-dimensional space looking down on the earth at the earth’s north pole (i.e., from the approximate direction of the north star Polaris), the moon orbits the earth in a counter-clockwise direction which is also the direction of both the moon’s and the earth’s rotation on their axes. It (counter-clockwise) is also the direction that the earth orbits the sun.

Since the moon’s orbit around the earth is in the same direction as the earth’s rotation on its axis (counter-clockwise), the moon, like the sun, always rises in the east and sets in the west. In reality, the rise and set of the moon are either somewhat north or south of east and west depending on the time of year.

Also, since the moon’s orbit around the earth is in the same direction as the earth’s rotation on its axis (counter-clockwise), and because the moon’s orbit takes 28 days while the earth’s rotation on its axis takes only one day, the moon moves toward the east each day so that the earth has to rotate a little longer each day to bring the moon into view from any given point on the earth’s surface. Thus, the moon rises, on average, about 50 minutes later each day. We can approximate how many minutes later each day the moon rises by multiplying 24 hours per day times 60 minutes per hour and dividing by 28 days to get approximately 51 minutes later each day.

Since the moon’s orbit around the earth is elliptical in shape, there are times during the moon’s orbit when it is closer to the earth than others. When a full moon or a new moon occurs at approximately the same time as when the moon is closest to the earth, it is called a supermoon because the lunar disk appears slightly larger than usual in size as seen from earth.

As previously stated, the moon orbits the earth once every 28 days. It also rotates on its axis once every 28 days. This is a situation known as synchronous rotation or tidal locking. Thus, only one side of the moon is ever visible from earth. The moon is illuminated by reflecting light from the sun, and the varying viewing conditions cause the lunar phases. At full moon, the moon rises at sunset and sets at sunrise. At third quarter, the moon rises at midnight and sets at noon. At new moon, the moon rises at sunrise and sets at sunset so that we don’t see any of the illuminated side of the moon.

For the moon to be visible during daylight hours, it must be in the sky at the same time as the sun, but not so close to the sun that we can’t see it. Since a full moon rises at sunset and sets at sunrise, we can’t see a full moon during the day.

The earth’s orbit around the sun is not in the same plane as the moon’s orbit around the earth. If both orbits were in the same plane, we would have a lunar eclipse and a solar eclipse every month. A lunar eclipse occurs when the moon enters the earth’s shadow. A solar eclipse occurs when the moon’s shadow falls on the earth. They do not happen every month because the earth’s orbit around the sun is not in the same plane as the moon’s orbit around the earth.

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